Ha! I had such a hoot doing this last year for my little girl. :) I found this great website for fun food pranks--Family Fun. There were so many fun ideas, but I decided to turn juice into gelatin and pre-slice a banana. She thought it was really fun, and, like I said, I had quite a fun time doing it also.
For the juice--just mix juice with Knox gelatin according to the package directions for "Knox Blocks" and pour it into a cup and let it set up. She really did pick it up and try to drink it. Hee hee...
Then for the banana, what you do is take a needle and thread and pierce into the banana. Then thread the thread around the banana (there's an image on the website) and pull it back through the same hole. The result is that it slices the banana but leaves the peel in tact. Then, when you peel the banana, it's already cut up. This also resulted in a very confused look and much snickering from me. :)
Anyway, it was a lot of fun. A little bit of work, but worth the smiles from her and chuckles from me.